The I band in striated muscle remains one of the least understood regions of the contractile unit, the sarcomere. Thin filaments are the major structural component in the I band and are in a tetragonal array at the Z band. Where the ends of the thin filaments penetrate into the A band to interact with thick filaments and to generate the contractile force, the thin filaments have hexagonal symmetry. How the transfer from tetragonal to hexagonal symmetry occurs and what happens to the arrangement and spacing of thin filaments in the I band is not known. Ultrastructural studies have identified N1 and N2 lines, regions in the I band which move in relation to sarcomere length. Biochemical studies reveal at least four new proteins in myofibrils, probably in the I band, one of which has been localized to the N2 line. The purpose of these studies is to determine the organization of I band components and to observe the related changes in thin filament ordering in normal adult and developing heart and skeletal muscle cells at different sarcomere lengths and in different physiological conditions. We will use electron microscopy at standard and high voltages combined with goniometer tilt and stereo methods and optical diffraction and optical reconstruction techniques. A three dimensional analysis of thin filament arrays in cardiac and skeletal muscle will give new information about I band structure and may suggest a role for some of the newly identified muscle proteins. We will determine 1) structural stability of some of the I band components, 2) how I band substructure is changed by the degree of overlap of thick and thin filaments, 3) how I band substructure is related to the variation in thin filament length that occurs in some muscles, 4) if further changes in I band structure occur in relation to structural changes in the adjacent Z band lattice and 5) if the changing variation in thin filament length in developing muscles is correlated with possible changes in the N regions of the I band. Studies of normal I band will add to our understanding of structure-function relationships in muscle cells in health and disease.